NZ247052A

NZ247052A - Packaging material for microwave oven - thermal barrier placed between paperboard substrate and printed susceptor-ink composition

Info

Publication number: NZ247052A
Application number: NZ247052A
Authority: NZ
Inventors: Roger D Hall; Christopher J Parks
Original assignee: Westvaco Corp
Priority date: 1992-03-04
Filing date: 1993-03-03
Publication date: 1995-04-27
Also published as: AU654642B2; JPH0632378A; US5231268A; NO930766D0; EP0559447A1; AU3400393A; CA2090838A1; NO930766L

Description

<div class="application article clearfix" id="description"> 2 7 APR 1995 WM NEW ZEALAND PATENTS ACT, 1953 No.: Date: COMPLETE SPECIFICATION PRINTED MICROWAVE SUSCEPTOR We, WESTVACO CORPORATION, a corporation of the State of Delaware, of 299 Park Avenue, New York, New York 10171, United States of America hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- - 1 - (followed by page la) //* ^ e 'i V f , ; b , * • / ' f - Background of Invention The present invention relates to susceptor packaging materials and packages constructed therefrom for use in heating foods in a microwave oven. The invention is an improvement in U.S. Patent Ho. 4,914,266 to Parks, and is related to the invention disclosed in U.S. Patent No. 5,132,144, granted July I'l, 1992, entitled 'Microwave Oven Susceptor', to Parks, who is a coi:. ventor cf the present invention. The construction of the present invention represents a further refinement in the prior inventions of Parks and produces a level of performance not achieved by the prior inventions. Current commercial microwave susceptor technology utilizes vapor deposited metallized films of aluminum on fil-that are laminated to paper or paperboard substrates. The metallized film technology is not readily adaptable to the application of susceptors in selected patterns nor can it be readily manipulated to control the level of heat generated in any one part of the susceptor substrate. The prior U.S. patent and pending patent application of Parks each address these problems and establish the viability of printed susceptors using known printing methods and equipment including oravure ,ind fl exoqraphy, offset and silk screen. The nerformance of r.ir.t ,u r.erii prepared from the susceptor paekaqinq materials of the Parks patent and patent application has been found to be comparable to metallized aluminum susceptors in the generation of heat and provides the flexibility of controlling both the location and ancur." rf r.eat produced oy tr.e suscap'cr. However. during tr.e deve 1 corner, t :: _r.cs5 inventions . it was oisccvera: "a" tr.e suostrata on vnicr. tr.e suscaptcr was printed could :ec::e degraded during cooking from tne generation c: excass teai iy tr.e suscsptcr. The generaticr. of excass heat increased the dancer cf fire or excessive sacking -ha" needed t- be minimised to provide a commercially acceptable product Acccrdingly, the present invention was developed to address tr.esa problems and to provide a commercially acceptable suscaptcr packaging material c: greater refinement, predictability and performance. The present invention is directed to susceotcr packaging material for a microwave oven that is prepared on printing press. Carbon black and graphite are conductive carbon materials that are available in particle sires which may be readily dispersed in printable ink vehicles. Inks incorporating conductive carbcr. materials can be printed or. paperbcard substrates to make susceptor packaging material useful in a microwave even. However, during experimental cocking tests, it was discovered that the paperbcard substrate in such packaging material could become degraded when exposed to microwave radiation as a result cf the generation of excess heat ty the susceptor. It is, therefore, an objec cf this invention to provide a thermal barrier between the t/y o} t,Sjo»,0 * Pel vest ar coated paperbcard has in "he past beer, r.e suostrate of chcics fcr ovenable packaging. It is pre--5 r r e d because of its FDA status and because it is readily :eat sellable fcr forcing food packages. A polyester coating ;r. paperbcard was found to provide thermal barrier protection ::r susceptor food packaging. However polyester coatings are -.ct compatible with ail types of susceptor coatings and, in :articular, are not readily compatible with tr.e preferred :rz.r.table susceptcr-ink composition of tr.e preser.t invention. Accordingly, in order to avoid these difficulties, a ncn-pclyester thermal barrier layer is preferred which will have good adhesion to the paperbcard substrate and also gcoc adhesion with the printed susceptor-ink layer of the susceptor packaging material. Sodium silicate is the preferred thermal barrier material of the present invention because it is compatible with the preferred susceptor-ink composition disclosed hereir and because it is thermally stable at temperatures far in excess of these needed for microwave susceptors. Sodium silicate is also FDA approved for food contact use, it can readily be applied to paperboard via coating, printing, or the like, and it is low in cost. Other thermal barrier materials useful in the present invention include polyesters " " • ■ V% 3 ■ . silicrr.es , urethanes, pclyimides , pclyamides , pclysul f cr.es . ether ir.crgar.ic silicates ar.d combinations of these materials. When ether thermal barrier coatings are used ir. the present invention, their surfaces may need tc be treated fcr geed adhesion with the preferred printable susceptor-ink composition (disclosed more fully hereinafter). Sodium silicates are compounds of silica (SiO,) and soda ash (Na,0) and are generally available as aqueous solutions. They may be readily applied to paperbcard by any well known coating process and may also be formulated so as tc be printed on paperboard with known printing methods. Such solutions are believed to be useful as thermal barrier layers because solutions containing silicates contain residual moisture when dried. It is known that sodium silicate will retain from 10-30% moisture depending upon how it is dried. This permits a thermal barrier coating containing sodium silicate to be applied by a conventional coating method or or. a printing press since all of the water in the solution need net be driven off during the drying step. Unfortunately this bcur.d water sometimes presents problems during microwave heating when the water vaporizes under the influence of excess heat. To counter this problem, the thermal barrier coating of the present invention may include an inorganic pigment such as clay, calcium carbonate or the like to create a degree of porosity sufficient to allow the escape of water ,'■<» * 't-v vapcr. Accordingly the present invention incorporates the printed susceptor technology originally disclosed in U.S. Patent No. 4,914,266, the improved susceptor-ink composition disclosed in U.S. Patent I.'o. 5,132, 144, and adds to that a thermal barrier layer between the substrate and susceptor layer more fully disclosed herein. Brief Description of the Drawing Figure 1 of the drawing illustrates in cross section a typical laminate for the susceptor packaging material according to the present invention. Detailed Description The present invention is directed to the manufacture ar.c use of susceptor packaging material that may be prepared on a printing press. The use of graphite or conduc tive carbon black in a printable susceptor material in the manufacture of susceptor packaging material for microwave ovens is known. The conductive carbon material is preferabl dispersed in an ink vehicle to produce a printable susceptor ink composition which is printed on a microwave transparent substrate using conventional printing technology. However, prior to printing the susceptor-ink composition on the substrate, the substrate is preferably coated with a thermal barrier layer to insulate the substrate from excess heat generated by the susceptor-ink layer when exposed to microwave energy. Finally the printed susceptor material is over- 5 structure c: tne composite susceptor packaging material according to the present invention. Reference character 14 represents the substrate onto which the susceptor-ink composition is printed during the manufacture cf the susceptor packaging material cf the present invention. Layer 14 is formed from a microwave transparent material such as a dielectric sheet material, e.g., paperbcard which provides the structural rigidity necessary fcr making packages or package inserts from the susceptor material. The upper surface cf the paperbcard substrate 14 may be coated cr ur.coated with tne understanding that the type of coating used could influence the thermal characteristics of the paperbcard, the type of thermal barrier coating used and the adhesion between the paperbcard, thermal barrier coating and susceptor-ink. coating. The lower surface is preferably coated with a clay containing coating tc provide a surface useful for printing graphics and other information about the use of the susceptor material cr the products packaged in packages made from the susceptor material. However, in some packaging applications, the lower surface or backside of the paperboar substrate 14 may be left uncoated if low quality or no graphics are needed, and to facilitate the escape^of water 6 vapcr :r:: -he surs-rj'e ur.cer severe .ieat:r.g :cr.c;";cr.s. Referring also tc Figura 1, rsfarar.ca cr.aractar 11 represents tr.e feed contact layer. This layer serves several purposes. Smca tr.e underlying layers say be :c;stur2 sensitive, layer 11 serves tc protect the underlying layers frcr zcistura per.etraticr. during stcrage and cocking. It alsc serves tc prctact the feed products packaged with the suscep-tcr ratarial frca possible ccr.taninants which sight aigrata frca the underlying layers. Release properties 2ay be incorporated into the food contact coating 11 to praver.t sticking of food products and in crder that the focc products ray be easily rarcved frcn the susceptor packages aftar cocking. Alternatively, a separate release coating (net shewn.) could be applied ever layer 11 if desired. Also, pigments 22ay be added to the focc contact coating if desired to produce a specific colored surface and other pigaer.ts ssay be incorporated to sake the layer porous for release of vater vapcr if needed. Suitable materials for use in the food contact layer 11 should be themally stable in excess cf 300 degrees and should aeet all FDA guidelines for contact with aqueous and fatty focds under all conditions experienced during packaging, storage and cocking. Examples of coatings for layer 11 include polyesters (Morton ADCCTE 3 3R2AK), acrylics (Goodrich HYCAR 26315), and silicones (Dew Corning SYL-OFF 7600). An example of an additional release cpAting that / / could be applied over the food contact layer 11 is QUILCN C free DuPcr.t. The food contact layer is applied either as a coating cr on a printing press in an amount of from aocut 2-25 lbs./ream (ream size 3,000 square feet). layer which is printed on the substrate 14. This layer provides the means whereby heat is generated when exposed tc microwave radiation for achieving microwave browning. Layer 12 comprises at least two components, an ink vehicle and an electrically conductive microwave interactive material. The preferred ink vehicle for the present invention is sodium silicate as disclosed in U.S. Patent No. 5,132,144. The sodium silicate binder preferably comprises 82-25 weight % of the electrically conductive layer at 40-55 % solids. The preferred microwave interactive material is a conductive carbon component, for example, graphite. Sodium silicate serves as a fire retardant binder for the microwave interactive graphite. As taught in the aforementioned pending application, sodium silicate has the thermal stability necessary for the intended application, unlike conventional printing ink binders such as polyesters, acrylics and nitrocelluloses. While polyesters and acrylics have been found to be suitable for the food contact layer 11, they have not demonstrated the thermal stability required for a binder in susceptor coating. Sodium silicate is available in weight ratios of silica to soda ash of from about 4:1 to about 1.5:1. The preferred ratio is on the order of about 3:1. A Layer 12 in Figure 1 designates the susceptor-ink 8 /" / • 4 »«• J i p i i :■ I * / v < J ci suitable scdium silicate is available from Occidental Chemical Company sold under the brand name 4 0 Clear :cr the purpose intended. Particulate graphite is available ir. a wide range of particle sizes, shapes, and purities. Fcr gravure printing, a particle size less than about 100 microns may be employed but less than about 10 microns is preferred. Examples cf graphites that have been successfully employed for susceptor coatings in the present invention include Superior Graphite 5539, having spherical particles of about 5 microns and a purity of 99.8% carbon, and Asbury Graphite Micro 250 with a particle size of about 0.5 micron. The ratio of graphite to sodium silicate solids in the susceptor-ink composition can range from about 15 to about 75% graphite by weight. As an example, a ratio of one part Superior Graphite 5539 and 3 parts sodium silicate 40 Clear, adjusted to a total solids of about 40%, and applied to a suitable substrate at about 20 lbs/3000 sq. ft. has been found useful lot browning microwave pizza. The electrically conductive layer is [!(•*(11*.ti) 1 y api lied over the thermal harrier coating in sufficient thickness to achieve a suiface resistivity of from about 107-10,000 ohm/sq. Layer 13 is the thermal insulator component of the present invention applied to the substrate 14 between the substrate and the microwave susceptor layer 12. Layer 13 is designed to provide a thermal barrier between the paperboard substrate 14 and the microwave susceptor layer 12 to prevent any degradation of the paperboard as a result of the generEr—-tion of excess heat by the susceptor when exposed to micro- 9 0,1 i wave radiation. Thermal barrier materials useful ir. tr.e present invention include polyesters, silicones, urethar.es, pclyizides, pclyamides, polysulf ones, scdium silioata ar.d other inorganic silicates and combinations therecf. The preferred thermal barrier is a coating containing sodium silicate which would be ccnpatible with the preferred susceptor-ink layer. If polyester is used as the thermal barrier 13, an extrusion coating of abcut 1.25 mil in thickness has been found to be useful. However, when polyester is used, proper adhesion of the preferred scdium silicate containing susceptor-ink conpositicn may net be easily achieved. Priming of the polyester surface with silar.es cr the addition of silar.es to the susceptor-ink composition has improved this adhesion somewhat. Flame or corona treatment cf the polyester surface has also improved this adhesion to the point where the susceptor-ink printing conpositicn may be successfully applied with a gravure application. With the use of sodium silicate as the thermal insulation layer, adhesion cf the preferred susceptor-ink layer is not a problem. First, sodium silicate will readily adhere to the surface of a paperboard substrate, particularly an uncoated substrate, and subsequent adhesion between the sodium silicate thermal layer and the sodium silicate containing susceptor-ink layer is no problem. Moreover a clay "AV>" coated paperbcard substrata will present ever, fever prcble-s tnar. unccated paperbcard because cf the tendency cf any aqueous solution tc scax into unccated paperbcard. It is also possible to use the polyester coated paperbcard normally used for ovenable packaging in the present invention, with an appropriate treatment as outlined above, if the polyester coating is net of sufficient thickness to serve as a true thermal protection layer. While it may be possible to use the same grade scdium silicate in the preferred thermal insulating layer 12 and as the binder in the preferred susceptor-ink composition layer 12, it may be preferable to use sodium silicates with different ratios of silica to soda ash in each layer. If it is desired to combine bcth polyester and sodium silicate as the thermal layer, flame treatment of the first down polyester coating will permit good adhesion to a second down sodium silicate thermal coating followed by the printed susceptor-ink layer containing scdium silicate. Wher. sodium silicate alone is used as the thermal barrier layer a solution having a solids content of from about 30-50% is preferred applied to the dielectric substrate in an amount cf from about 6 to 24 lbs./ream (ream size 3000, square feet). Another feature of the present invention involves the selective pigmentation of one or more of the food contact layer 11, susceptor-ink composition layer 12 or the thermal 11 barrier layer 12. Scdium silicate is Jcr.cwn tc retain a large amount or bound water, particularly when dried at tr.e temperatures experienced on a gravure press vnich are lever than those that would be experienced during microwave cocking. Likewise the scdium silicate in the thermal barrier layer wculd retain moisture. Obviously some cf this bound water is likely to be released when the susceptor layer 12 is heated to temperatures in excess of 300 degrees F. upon exposure to microwave radiation. As this moisture is liberated, it can produce pinholes and voids in the layers which reduces the rub resistance of the coatings after cocking. To counter this effect, bcth the food contact coatir.c applied as layer 11 and the thermal barrier coating applied as layer 13 may be pigmented with clay, calcium carbonate or titanium dioxide or other non-microwave interactive piqments to create a somewhat porous structure which allows the escape of water vapor from the coatings without reducing their rob resistance. Since the interactive layer 12 is already pigmented with graphite, the escape of moisture from this layer does not present a problem in most cases, besides, the addition of non-microwave interactive materials to the microwave interactive layer would reduce its efficiency. However, in extreme cases, some additional pigmentation could be added to the microwave interactive layer 12. \ 2. </div>

Claims

<div class="application article clearfix printTableText" id="claims"> WHAT CLAIM IS

1. A composite susceptor material for the generation of heat, by absorption of microwave energy comprising: (a) a porous, dielectric, substrate substantially transparent to microwave radiation; (b) a thermal barrier layer on one surface of said substrate; (c) an electrically conductive layer printed on the surface of said thermal barrier layer, said electrically conductive layer comprising a susceptor-ink composition of a microwave interactive material dispersed in a binder and capable of being applied by a printing press; and, (d) a product contact layer applied over the electrically conductive layer having characteristics suitable for the safe packaging of food products.

2. The composite susceptor material of claim 1 wherein the • ■ 11■;'t i w.i 1 1 y {-oiviuct lvu layer c-onvariscs from substantially l'j-7'J weight percent finely divided conductive carbon and from 85-25 weight percent sodium silicate binder at 40-55% solids.

3 . The composite susceptor material of claim 2 wherein the electrically conductive layer is applied over the thermal barrier coating in sufficient thickness to achieve a surface resistivity of from ant lal ly 1^7-ln,',)00 ohn/sq. > <• 13 <Lu

4. The composite susceptor aaterial of claim 3 wherein the tneraal barrier layer comprises an insulating aaterial selected from the group consisting of polyester, silicones, urethanes, polyimides, polyanides, polvsulfones and inorganic silicates.

5. The composite susceptor material of claim 4 wherein the thermal barrier layer comprises a sodium silicate coating havinu a solids content ot from substantially id to in)*, and is applied to the dielectric substrate in an amount of from sui. stantially 6 to 24 lbs/ream.;6.

The composite susceptor aaterial of claim 5 wherein the product contact layer comprises a material selected from the group consisting of polyesters, acrylics and silicones and is applied over the electrically conductive layer in an amount ot t roin substantial 1 y 3 to 25 lbs. /ream.;7.

The composite susceptor material of claim 6 wherein the thermal barrier layer further comprif.es up to substantially 50 weicjht percent inorganic pigment selected from the group consisting of clay, calcium carbonate and titanium dioxide.;8.

The composite susceptor material of claim 7 wherein the porous, dielectric substrate is selected from the group consisting of uncoated paperboard, paperboard having a pigmented coating and paperboard containing a surface coating of a polyester material.;14 -;9 .

The composite suscept:r material cf claim 3 vr.ersin the focc ccr.wac" layer further comprises a release agent.;10.

The composite susceptor aaterial cf claim 3 wherein the feed contact layer is cverccatec with a coating containing a release agent.;11.

A process of manufacturing a ccmccsite susceptor material for the generation of heat by absorption cf microwave energy comprising:;(a) providing a porous, dielectric, substrate substantially transparent to microwave radiation;;(b) applying to cne surface cf said substrate a thermal barrier layer for insulating the substrate from excess heat generated by microwave energy;;(c) printing cn said thermal barrier layer a susceptor layer of a susceptor-ink composition comprising a dispersion of finely divided, electrically conductive microwave interactive particles suspended in a printable ink vehicle; and,;(d) applying over said susceptor layer a protective layer having characteristics suitable for safe contact with fatty foods and the like.;12.

The process of claim 11 wherein the susceptor layer (c) is printed on the thermal barrier layer with a printing press in a pattern and with varying thickness to generate varying;* ' V " ' / \ , v '•/ ' 15 decrees of heat throughout the susceptor material vnen exposed to microwave energy. 1j.

The process of claim 12 wherein the susceptor layer (c) is printed on said thermal barrier layer with a press selected from the group consisting of gravure, offset, flexographv, and silkscreen.

14. A disposable microwave food heating container adapted to accommodate one or more food products comprising an outer container body formed from a microwave transparent, dielectric substrate said substrate having printed on one surface thereof a susceptor-ink composition comprising microwave interactive particles suspended in a printable ink vehicle in a preselected pattern corresponding to the location of the food packaged in the container, and a food contact coating applied over the printed susceptor-ink composition to provide a food contact surface for food packaged in the container, ,i thermal barrier layer beinq provided between the microwave transparent dielectric substrate and the printed susceptor-ink composition to insulate the substrate from excess heat generated by the printed susceptor-ink composition when the container and its food products are heated in a microwave oven.

15. The food heating container of claim 14 wherein the thermal barrier layer comprises an insulating materal selected 16 from the group consisting of polyesters, silicones, urethanes, polyiaides, pclvamides, polvsulfones and morgan silicates.

16. The food heating container of claim 15 wherein the theraal barrier layer is prepared from a sodium silicate solution havmq a solids content of Iron1, substantially 30 to 5''"i which is applied to the dielectric sheet material in an amount Of from substantially (■ to .M lbs./ream.

17. The food heating container of claim 16 wherein an inorganic pigment is incorporated into the thermal barrier layer in an amount of up to substantially 50 weight percent to provide voids in the layer for releasing bound moisture associated with the sodium silicate during the microwave heating process.

18. The food heating container of claim 17 wherein the susceptor ink composition printed on the thermal barrier layer comprises from substantially 15-75 weight pcrccnt finely divided conductive carbon particles suspended in a binder o fmm substantially 05-25 weight percent sodium silicate at 40-55% solids.

19. The food heating container of claim 18 wherein the thic ness of the susceptor-ink composition is varied within the preselected pattern printed on the thermal barrier layer to provide varying degrees of heat for the food products pack- 17 agec ir. sa ia container wr.er. exposed tc microwave energy.

20. The feed heating container cf clai- IS wnereir. the feed contact coating comprises a aaterial selected from the group consisting of polyesters, acrylics and silicones and is applied ever the susceptor-ink composition printed on the ma 1 barrii-i m an amount »•:' ro:•. substantially 3 to 25 lLs. /rear..

21. The food heating container cf claim 20 wherein a release Kiei.i is included in the food contact coating or applied over the food contact ccating to keep the food products packaged in the container from sticking to the container.

22. The food heating container of claim 21 wherein the microwave transparent, dielectric substrate is paperboard.

23. A composite susceptor material for the generation of heat by absorption of microwave energy substantially as herein described with reference to the accompanying drawing.

24. A process of manufacturing a composite susceptor material for the generation of heat by absorption of microwave energy substantially as herein described with reference to the accompanying drawing.

25. A disposable microwave food heating container substantially as herein described with reference to the accompanying drawing. 18 </div>